This invention relates to a variable ratio steering gear of a recirculating ball type and particularly to improvements in a ball nut having a rack and a sector gear having tapered teeth meshable with the rack, which are incorporated in such steering gear.
As is well known in the art, a variable ratio steering gear is used on automotive vehicles of various types and is constructed to provide a high steering ratio at the neutral position of the steering wheel and a decreasing ratio as the wheel is displaced from the neutral position.
It has been proposed in the art to construct this kind of steering gear as shown in FIG. 1.
Referring to FIG. 1, a steering shaft 1 is provided with a helical groove 2 and is rotatably supported by a housing 3. The steering shaft 1 is connected to a steering wheel (not shown) and is rotatable together therewith. A ball nut 4 having an internal helical groove 5 is fitted over the helically grooved portion or the worm portion 2 of the steering shaft 1 and is disposed slidably within the housing 3. A plurality of balls 4 are disposed in a helical ball circuit (not designated) defined between the helical grooves 2 and 5 of the steering shaft 1 and the nut 4 so that the nut 4 may move axially of the steering shaft 1 upon rotation of the shaft 1. On the lower side of the nut 4 is formed a rack 6 having two outside teeth and two inside teeth located between the outside teeth. The teeth of the rack 4 are generated such that the top lands or tops 6b of the outside teeth are located higher, i.e. projected more outwardly of the nut 4, than the tops 6a of the inside teeth. A rockshaft 7 is rotatably supported by the housing 3 and is located transversely to the steering shaft 1 and the worm portion 2 thereof. The rockshaft 7 has integrally formed therewith a sector gear 8 the teeth of which mesh with the teeth of the rack 6. The teeth of the sector gear 8 are generated in a manner to have top lands or tops lying on an addendum curve 9 which is made up of a single arc having a constant radius r.sub.k and a center coinciding with the center O of rotation of the sector gear 8. The teeth of the rack 6 have various pressure angles. That is, the pressure angle of the inside tooth surfaces 6c of the inside rack teeth is designed sufficiently large for the convenience of avoiding interference between the teeth of the sector gear 8 and the rack 6. The pressure angle of the outside tooth surfaces 6d of the inside rack teeth is designed larger than that of the inside tooth surfaces 6c so as to prevent creation of undercuts in the roots of the teeth of the sector gear 8. The pressure angle of the inside tooth surfaces 6e of the outside rack teeth is designed considerably small so as to attain smooth and continuous engagement between the teeth of the sector gear 8 and the rack 6.
In this structure thus described, due to the fact that the radius of the pitch circle of the sector gear 8 is adapted to decrease in accordance with the increase in the displacement of the steering wheel from its neutral position and that the addendum curve 9 is a circular arc having a center located on the center O of rotation of the sector gear 8, the central tooth of the sector gear 8 has a tooth profile of the kind similar to the minus-shifted tooth profile in the theory of profile shifted gears, that is, the central sector gear tooth has tooth surfaces extending to intersect each other at a relatively sharp angle, whereas the outside teeth of the sector gear 8, which are located at each side of the central sector gear tooth, have tooth profiles of the kind similar to the plus-shifted tooth profile in the theory of profile shifted gears, that is, the outside sector gear teeth each has tooth surfaces extending to intersect each other at a blunter or less acute angle as compared with the central sector gear tooth. FIG. 2 shows the side elevation of the rockshaft 7 and the sector gear 8 incorporated in the variable ratio steering gear of FIG. 1. As shown in FIG. 2, the teeth of the sector gear 8 are tapered in the axial direction thereof, so the tooth profiles of the sector gear at the larger diameter end are tapered at the tops more acutely as compared with the corresponding tooth profiles at the smaller diameter end. Accordingly, the tops of the teeth of the sector gear 8, particularly the tops 8b and 8c of the outside teeth thereof are liable to be pointed or tapered considerably at the larger diameter end of the sector gear 8. The sector gear 8 having such pointed tops 8b and 8c is inferior in strength as well as engagement efficiency. In order to prevent creation of such pointed tops 8b and 8c, reduction of the radius of the addendum circle 9 may be considered useful so as to cut off the pointed top portions which are excluded by the reduced radius of the addendum circle. This, however, tends to cut off the tops 8a of the central tooth excessively, which inevitably involves an insufficient contact ratio in the engagement between the teeth of the sector gear 8 and the rack 6.
Furthermore, as the tops of the teeth of the sector gear 8 are located on a circular arc having a center located on the center of rotation of the sector gear 8, the teeth of the rack 6 are formed to have bottoms 6f and 6g lying on a common straight line extending parallel to the axis of the steering shaft as shown in FIG. 1. Accordingly, the minimum distances h from the internal helical groove 5 to the respective bottoms 6f and 6g are equal to each other. This involves a drawback in that the ball nut 4 has less strength at the localized portions thereof corresponding to the positions where the bottoms 6g are closest to the helical groove 5, because at the above localized portions a relatively large stress is induced in the nut 4 when the steering wheel is in a maximum steering angle condition. Thus, when the vehicle is driven to ride over a curb with the steering wheel on the maximum lock, a considerably large stress is induced in the localized portions of the nut 4 near the bottoms 6g and can cause a crack or cracks of the nut 4 near the bottoms 6g of the rack teeth. The nut 4 formed with such rack teeth is thus undesirable as a safety part of the vehicle.
The foregoing drawbacks existing in the prior art variable ratio steering gear of the kind thus described can be met by the structure disclosed in the U.S. Pat. No. 2,953,932 in which the outside teeth of the sector gear are formed smaller than the central tooth, i.e., the outside teeth are of smaller module as compared with the central tooth. However, the steering gear of U.S. Pat. No. 2,953,932 is expensive because its sector gear blank can not be produced or finished to the size through turning but requires complicated and therefore costly machining.